In investigative logging, the ability to determine phase transition pressure is important because the presence of a phase transition may render invalid a fluid sample taken for the purpose of composition analysis. In production logging, it is necessary to know the phase transition pressure because phase separation limits production flow rate.
A sample containing retrograde condensates is typically found when a fluid sample is taken from a deep hot oil and gas reservoir by wireline fluid sampling (WFS). Retrograde condensates are susceptible to downhole phase separation, including dew precipitation and bubble creation. Phase separation downhole in oilfield fluids from a particular formation limits the “drawdown” pressure drop that can be used in production of crude oil from that formation, thereby limiting the flow rate of crude oil production from that formation. Phase separation in oilfield fluids is separation into a continuous phase and a discrete phase, either into a gas phase and a liquid phase, or into a continuous liquid phase and a discrete liquid phase. Phase separation is of particular concern when attempting to obtain valid samples from deep hot oil and gas reservoirs. For a sample to be valid, it must be of single phase.
When dew precipitation occurs in a downhole pipe or flow line, dew typically forms first as a mist. After dew forms as a mist, most of the dew deposits on the walls of the containing pipe in the form of film. Liquid enriched in heavier ends can flow as an annular film on the inner surface of the pipe or flow line, with gas or depleted condensate entrained in the middle of the pipe or flow line. Sometimes mist coalesces into slug form.
The phase diagram of FIG. 16 (prior art) shows a pressure reduction (represented by arrow 77) taking the fluid from single-phase domain 71 into multi-phase domain 72 to produce dew precipitation as the dew precipitation transition boundary 74 is crossed. Dew precipitation transition boundary 74 is the single-phase/multi-phase boundary between critical point 75 and cricondentherm 79. Bubble creation transition boundary 73 is the single-phase/multi-phase boundary at lower pressures and temperatures bounded by critical point 75. Bubbles are created when transition boundary 73 is crossed by a pressure reduction (represented by arrow 76) taking the fluid from single-phase domain 71 into multi-phase domain 72.
Retrograde condensates are defined by having formation conditions where the formation temperature is between the fluid critical point and the cricondentherm. At the critical point of a fluid (“critical point” is a term used in thermodynamics), distinction between gaseous phase and liquid phase ceases to exist. In most situations, formation fluid has one and only one critical point in the whole pressure and temperature range. The “cricondentherm” is the highest temperature in which dew is still able to precipitate out of mixture, and where the reservoir pressure is above the dew line. FIG. 16 shows critical point 75 and cricondentherm 79. The pressure reduction used for fluid sample extraction can cause dew precipitation. If dew precipitation occurs, it can take the form of a mist, thin film, or relatively large slug, depending on sampling conditions. The phase separation is usually from single-phase to two-phase. (More rarely 3 or 4 phases can occur). In the phase diagram of FIG. 16, a pressure reduction would appear as a descent from the single-phase domain 71 into the multi-phase domain 72. Phase transition pressure is the formation pressure at boundary 73 or 74.